146 
PALAEONTOLOGY OF NEW YORK . 
1884. Crania, White. Thirteenth Rept. State Geologist Indiana, p. 121. 
1S84. Crania, Spencer. Bull. No. 1, Mus. Univ. State of Missouri, p. 57. 
18S6. Crania, Ringueberg. Bull. Buffalo Soc. Nat. History, vol. v, pp. 16, 17. 
1889. Crania?, Walcott. Proceedings United States National Museum, 1888, p. 441; Advance sheets. 
1889. Crania, Beecher and Clarke. Memoirs N. Y. State Museum, vol. i, No. 1, p. 18. 
1889. Crania, Nettelroth. Kentucky Fossil Shells, p. 2. 
Diagnosis. Shell inequivalve, inarticulated, without perforation for a pedicle; 
subcircular in outline, generally somewhat transverse across the posterior mar¬ 
gin; attached by the apex or the entire surface of the lower valve. Ventral 
or lower valve depressed-conical or conforming to the surface to which it is 
attached. Dorsal or upper valve more or less conical with a subcentral, poste¬ 
riorly directed apex. External surface of the valves usually smooth, sometimes 
spinose or with concentric or radiating striae. In the interior of both valves 
are two pairs of large adductor scars, the posterior of which are close upon the 
margin and widely separated, the anterior near the center of the shell and 
close together, more approximate in the lower than in the upper valve. These 
posterior scars are often strongly elevated on a central callosity which sur¬ 
rounds their anterior margins. The margin of the lower valve is usually broad 
and thickened. Impressions of the pallial genital canals coarsely digitate. 
Shell-substance calcareous; strongly punctated by vertical canals which be¬ 
come subdivided toward the epidermal surface. 
Type, Crania craniolaris, Linne. 
Observations. Crania is remarkable for presenting an association of shell- 
characters which have undergone no essential change from the earliest known 
appearance of the genus in Lower Silurian faunas to the present. Indeed 
neither palaeozoic nor recent species indicate material variation from the type 
of internal structure found in C. craniolaris, while certain Mesozoic forms (C. 
Parisiensis, Defrance, from the Jurassic, C. tripartita, Munster, of the Cretaceous, 
etc.), give evidence of so great departure from the type in the development of 
internal diverging septa, in one or both valves, that separate subdivisions have 
been established for their reception; viz., Ancistrocrania and Craniscus, Dali, 
respectively. 
The degree of attachment of the lower valve has been made a basis of sub- 
